It is generally believed among energy experts that one of the last frontiers in saving energy in a global scale lies in the area of indoor space ventilation, particularly in commercial buildings. As a matter of fact, most experts predict that as much as ⅔ of the energy needed within the next 30 years will come from conservation technologies. Today, it is recognized that a vast amount of energy is being wasted to condition incoming air for mandated maximum occupancy ventilation according to ASHRAE standards, when in fact such spaces often have fewer than the maximum occupancy or no occupants at all. The difference in the level of ventilation actually needed versus the amount mandated by codes assuming full occupancy of the space represents the potential energy savings.
Carbon dioxide (CO2) gas and water are known to be the end products of energy deriving metabolism of food in humans and many animals. An average human breathes out at the end of each breath (called the “end-tidal” portion) up to 5% of CO2 or 50,000 ppm in concentration and the average CO2 level in an exhaled breath hovers around 3,000-5,000 ppm or 0.30-0.50% of CO2 by volume. The average outdoor CO2 level is on the other hand typically only around 400-700 ppm. Consequently, CO2 has been known for quite a while as an excellent surrogate parameter associated with the presence of humans. Thus when one or more persons are present in an enclosed space, the average concentration level of CO2 in said space will increase steadily until outside air is brought into the space. The object of ventilating an enclosed space like many modern offices in high-rise buildings is to bring in fresh air from the outside that has a much lower CO2 concentration in order to prevent the CO2 level buildup from exceeding 1,000 ppm, which is mandated to be the ASHRAE standard. Until about a decade ago, a CO2 analyzer capable of accurately measuring CO2 concentration down to ppm levels cost several thousands of dollars. Today, a good CO2 sensor with equal efficacy still costs upwards of several hundred dollars.
The method of using CO2 concentration level to detect the presence of humans in an enclosed space, thereby bringing in just enough outside air to keep the CO2 level under 1,000 ppm at all times (per ASHRAE standards), is commonly referred to as the Demand Control Ventilation (DCV) strategy. Depending upon what climate is prevalent in a particular geographical locale, such a strategy can lead to significant energy savings even in moderate climate zones. Although DCV strategies have been known for a number of years or ever since the low cost CO2 sensors became available in the marketplace, such an energy-saving ventilation strategy has not been widely practiced in retrofitting old or existing commercial buildings because of one additional barrier. In order to exploit DCV, one has to install a number of CO2 sensors in certain specific locations and in addition “wire” these sensors back to the Building Automation System (BAS) through one or more so-called Gateway controllers. For retrofitting old or existing commercial building, often times such installation and wiring costs far exceed those for the hardware (CO2 sensors and Gateway controllers) and software (new or modified software for the BAS). Consequently, on the average, it would require at least three or more years to recover, through annual energy savings, the initial investment in implementing the DCV strategy. Thus it becomes apparent that in order for existing commercial building owners or operators to buy into the use of DCV to save energy, the overall outlay for DCV systems, particularly the installation and wiring part, needs be drastically reduced.
As alluded to above, it is quite apparent that without a DCV strategy, over-ventilation of a space would lead to unnecessary energy expenditures. Even more importantly, in certain specific locations such as the southern states in the U.S., particularly in the state of Florida, where the climate is invariably hot and humid for a good portion of the year, indiscriminant over-ventilation could lead to another often ignored peril that can drastically affect the indoor air quality (IAQ) of a space. That is because moisture-laden air, if not properly conditioned to remove its water content, can lead to the buildup of molds over time in inaccessible areas of a building without ever being noticed until a serious IAQ problem surfaces. In recent years, many building IAQ problems have been attributed to, to the surprise of many industry experts, the presence of molds in inaccessible areas of the building such as air ducts, wall partition spaces, basements etc. because of excessive moisture in the air. The principal culprit of molds buildup has now been identified as the over-ventilation and inadequate conditioning of moisture-laden outside air which has to be brought in for meeting the 20 cu. ft. per occupant per minute building code standard. It is therefore prudent that in addition to the CO2 sensor, sensors for monitoring Relative Humidity (RH) and temperature (T) should also be included in the deployment of DCV strategy to save energy for existing commercial buildings while at the same time safeguarding the buildup of molds in inaccessible areas and not compromising the maintenance of the overall good IAQ.